Eduqas Biology for A Level Book 1: 2nd Edition

Tertiary structure : The α -helix of the secondary protein structure can be folded and twisted to give a more complex, compact 3D structure. This is the tertiary structure. The shape is maintained by: ▪ Hydrogen bonds. ▪ Ionic bonds. ▪ Disulphide bonds. ▪ Hydrophobic interactions. These bonds are important in giving globular proteins, e.g. enzymes, their shape. Quaternary structure : Some polypeptide chains are not functional unless they are in combination. In some cases, they may combine with another polypeptide chain, such as the insulin molecule, which has two chains. They may also be associated with non-protein groups and form large, complex molecules, such as haemoglobin. Globular and fibrous proteins The roles of proteins depend on their molecular shape. ▪ Fibrous proteins have long, thin molecules and their shape makes them insoluble in water, so they have structural functions, as in bone. The polypeptides are in parallel chains or sheets, with many cross-linkages forming long fibres, for example keratin, the protein in hair. Fibrous proteins are strong and tough. Collagen is a fibrous protein, providing the strength and toughness needed in tendons. A single fibre, sometimes called tropocollagen, consists of three identical polypeptide chains twisted around each other, like a rope. The three chains are linked by hydrogen bonds, making the molecule very stable. ▪ Globular proteins are compact and folded into spherical molecules. This makes them soluble in water and so they have many different functions, including enzymes, antibodies, plasma proteins and hormones. Haemoglobin is a globular protein, consisting of four folded polypeptide chains, at the centre of each of which is the iron-containing group, haem. Test for protein – the biuret test To test a solution for protein, a few drops of biuret reagent (sodium hydroxide and copper (II) sulphate) are added, although they can be added separately. The sodium hydroxide and copper sulphate react to make blue copper hydroxide. If a protein is present, the copper hydroxide interacts with the peptide bonds in the protein to make biuret, which is purple. So, the colour change for a positive biuret test is blue -----> purple. At low protein concentration, the colour change is difficult to detect by eye. The more concentrated the protein, the darker the purple colour, so the test is qualitative. It could be used as a semi-quantitative test, comparing the intensity of purple in two identically treated solutions. Measuring the absorbance of the purple biuret in a colorimeter, using a yellow (580 nm) filter gives a numerical estimate of relative concentration of proteins present in a sample. This is also semi-quantitative as an actual protein concentration is not measured. To measure the actual concentration, making the test quantitative, a biosensor is needed. Bond type Level of protein structure Primary Secondary Tertiary Peptide 4 4 4 Hydrogen 4 4 Ionic 4 Disulphide 4 Hydrophobic interaction 4 Stretch & challenge The R groups of some amino acids contain –CH 3 and –C 2 H 5 groups, which are hydrophobic. These groups cluster in the middle of the protein molecule. Their positioning together, away from water molecules, is called hydrophobic interaction. Stretch & challenge The haem group is in the class of non-protein molecules called porphyrin rings. In chlorophyll, a porphyrin ring also combines with protein, but the metal ion it contains is magnesium, not iron. Exam tip Make sure you can explain how to perform and interpret the test for proteins. Exam tip Learn by heart the four ways that the tertiary structure of proteins is maintained: hydrogen, ionic and disulphide bonds and hydrophobic interactions. Link The structure of haemoglobin is shown on p13. Knowledge check Match the macromolecules with their subunits: A. Glycogen B. Phospholipid C. Haemoglobin D. Cellulose 1. Amino acids + haem 2. α -glucose 3. Glycerol + fatty acids + phosphate 4. ß-glucose 1.8 Fi 15 3 polypeptide chains tightly wound around one another Chemical bonds and protein structure The structure of collagen, a fibrous protein 27  1.1 Chemical elements and biological compounds